Clear, moisture curable sealants are useful in many applications. The most widely used clear moisture curable sealants are silicon moisture curable sealants. However, such moisture curable sealants are expensive. Transparent polyurethane sealants exist, but have poor weathering resistance so would not be practical for outdoor use. It would be desirable to make a lower cost moisture curable sealant that is clear and weatherable. Polyolefins offer such a promise, in particular, hydrolizable silane graft polyolefins, as these could potentially be made into compositions that are easily moisture curable and would give very good adhesion to polar substrates including glass.
Silane graft polyethylene-based compositions (having at least 50 wt % ethylene derived units) have been disclosed in EP 0 510 559 A2, WO 2005/003199, U.S. Pat. No. 6,472,015, U.S. Pat. No. 6,455,637, U.S. Pat. No. 5,844,009, U.S. Pat. No. 5,883,145, US Patent Publication 2002/0151647 and US Patent Publication 2003/0050401. One problem with using polyethylene-based polymers in forming graft polymers is that the peroxides that are used to initiate the grafting also act as a cross-linker towards polyethylene themselves. Thus, what results from contacting peroxides with polyethylene-based polymers is a highly cross-linked polymer with higher molecular weight, and thus very high viscosity, specifically in the case where a high silane level (which necessitates the use of high levels of peroxides) is required to get higher performance. Such compositions are not suitable for moisture curable sealants. Further, the cross-linked polyethylene tends to lack clarity due to its crystalline nature.
On the other hand, peroxides tend to act as chain-breaking agents towards polypropylenes. S. Yang et al., “Mechanism of a One-Step Method for Preparing Silane Grafting and Cross-linking Polypropylene” in 47 POLYMER ENGINEERING AND SCIENCE 1004 (2007) teaches the grafting of silane to polypropylene, thus forming moisture cross-linkable propylene homopolymer. However, this article teaches grafting and cross-linking in one step. To be useful as a moisture curable sealant, the graft polymer itself is preferably stable and fluid, or of low viscosity, prior to its application to a surface as a sealant.
The grafting of maleic anhydride to propylene copolymers having high clarity has been disclosed in U.S. Pat. No. 6,884,850 and US 2008-0299397 A1, where it is taught to graft a propylene-ethylene copolymer with ethylenically unsaturated carboxylic acids or acid derivatives or epoxides (e.g., maleic anhydride) by the reaction of the moiety and a peroxide with the polymer. However, such a composition is not moisture cross-linkable, hence, not suitable as a curable sealant.
What would be desirable is a clear, moisture curable sealant (being stable and having a low viscosity prior to curing) comprised mostly of polyolefinic material, thus reducing cost. The inventors have discovered a solution to this problem.